The present invention relates to an oil pump assembly, particularly an oil pump assembly having a pair of pumps and a control unit for selectively supplying a fluid under pressure from the oil pumps to a fluid-pressure-actuated device.
Power steering systems are installed on automobiles to reduce the driver's effort needed to turn heavy steering wheels. The hydraulic source used in the power steering system comprises a pump driven from the automobile engine. The amount of a fluid discharged under pressure by the pump varies in proportion to the RPM of the engine. Therefore, it is required that the pump have a capacity to supply the fluid at a rate large enough to operate the fluid-pressure-actuated device such as a power steering gearbox even when the engine rotates at low speeds, that is, when the amount of the fluid discharged from the pump is small.
With such a pump capacity setting, the pump will supply the fluid at an unnecessarily large rate when the engine rotates at higher speeds. The pump capacity therefore results in unnecessary circulation of the fluid through the power steering system. The engine power consumed for driving the pump is increased to the point where the fuel economy of the engine is adversely affected. The foregoing pump arrangement is undersirable from the standpoint of energy saving.
To avoid such a difficulty, there has heretofore been proposed a pump assembly including two small-capacity pumps combined with a control unit for selectively supplying a fluid from the pumps to the fluid-pressure-actuated device. Under normal conditions, only one of the pumps is in operation to supply a fluid pressure, while the other pump is connected to a fluid tank under no load, thereby reducing the power consumption requirement. When need arises, the control unit is operated to enable both of the pumps to discharge pressurized fluid flows that are combined and supplied to the power steering gearbox. Various means are known for controlling the amount of fluid to be supplied under pressure. One control means comprises an engine RPM sensing system in which the rate of fluid flow is detected when the engine rotation is slow or when the amount of fluid discharge from the pump is small, for enabling the two pumps to produce a combined flow of fluid under pressure. According to a pressure-sensitive system, the two pumps are actuated to supply a combined fluid flow in response to detection of a pressure build-up developed in a fluid supply passage only when the power steering gearbox is operated or put under load irrespective of the engine RPM. Still another system is a combination of the above two systems, incorporating the advantages thereof. These known systems are selectively used to suit a particular application. The foregoing control unit is required to have the passage switching function to effect selective switching between the passages leading from the pumps, and the flow rate controlling function to keep the rate of fluid flow to the power steering gearbox below a predetermined level. It is general practice to perform these functions by using a pair of spool valves and fluid passages combining the spool valves.
It is important in constructing the oil pump assembly of the type described that the two pumps, two spools, and interconnecting passages be assembled in a single pump body in a manner to allow efficient operation, and the parts be machined and assembled easily for achieving a reduction in the manufacturing cost.
Accordingly, there is a need for oil pump assemblies which are simple in construction, can be assembled with ease, are small in size and lightweight. These requirements are particularly demanded for power steering units that will be mounted in a small space such as an engine room.
To meet the above requirements, it is generally necessary that the assemblage of the pumps on a common drive shaft in the pump body be simplified, taking into account the structural relationship between the pumps, the fluid passages from the pumps, and the spool valves for controlling the flow of the fluid. The control unit widely varies in structure from sensor system to sensor system. It is therefore desirable that the parts, especially the pump body, be constructed in order to be shared by different control unit structures, for thereby increasing the mass production capability.